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From: =?UTF-8?Q?Fran=c3=a7ois_Pottier?= <francois.pottier@inria.fr>
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Date: Wed, 4 Jan 2017 14:08:59 +0100
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Subject: [Caml-list] ppx_deriving question: deferring code generation?


Hello all,

I am currently in the process of writing a ppx_deriving plugin, called
"visitors". Overall, this has been a pleasant experience; a few hundred 
lines
of code have been sufficient to obtain nontrivial results.

In normal use, the user writes something like this:

   type foo =
     Bar | Baz
     [@@deriving visitors]

and some generated code is inserted just after the definition of the 
type foo.

However, I have reached a situation where the generated code cannot be 
placed
just after the type definition. That is, I need to allow user-written 
code to
appear after the type definition and before the generated code.

For instance, this user-written code could be a declaration of a global
variable "x", whose type is "foo ref", and which the generated code 
uses. The
declaration of "x" must appear after the definition of the type "foo", 
because
the type of "x" mentions "foo". And the declaration of "x" must appear 
before
the generated code, because the generated code (intentionally) refers to 
"x".

I am imagining that perhaps the user could write something like this:

   type foo =
     Bar | Baz
     [@@deriving visitors { delayed = true }

   let x : foo option ref =
     ref None

   [@@visitors]

The effect of the flag { delayed = true } would be to store the 
generated code
somewhere in memory (instead of emitting right away), and the effect of the
floating attribute [@@visitors] would be to fetch that code from memory and
emit it.

To me, this seems somewhat ugly, but workable. Does ppx_deriving offer a
better approach? Does anyone have a better suggestion? Comments are
appreciated.

Best regards,

--
François Pottier
francois.pottier@inria.fr
http://gallium.inria.fr/~fpottier/